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AUSCERT External Security Bulletin Redistribution
Cisco IOS XE Software DNS NAT Protocol Application Layer
Gateway Denial of Service Vulnerability
25 March 2021
AusCERT Security Bulletin Summary
Product: Cisco IOS XE Software DNS NAT Protocol
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Denial of Service -- Remote/Unauthenticated
CVE Names: CVE-2021-1446
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Cisco IOS XE Software DNS NAT Protocol Application Layer Gateway Denial of
Advisory ID: cisco-sa-alg-dos-hbBS7SZE
First Published: 2021 March 24 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvv65113
CVE Names: CVE-2021-1446
o A vulnerability in the DNS application layer gateway (ALG) functionality
used by Network Address Translation (NAT) in Cisco IOS XE Software could
allow an unauthenticated, remote attacker to cause an affected device to
The vulnerability is due to a logic error that occurs when an affected
device inspects certain DNS packets. An attacker could exploit this
vulnerability by sending crafted DNS packets through an affected device
that is performing NAT for DNS packets. A successful exploit could allow an
attacker to cause the device to reload, resulting in a denial of service
(DoS) condition on an affected device.
The vulnerability can be exploited only by traffic that is sent through an
affected device via IPv4 packets. The vulnerability cannot be exploited via
Cisco has released software updates that address this vulnerability. There
are no workarounds that address this vulnerability.
This advisory is available at the following link:
This advisory is part of the March 2021 release of the Cisco IOS and IOS XE
Software Security Advisory Bundled Publication. For a complete list of the
advisories and links to them, see Cisco Event Response: March 2021
Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled
o Vulnerable Products
This vulnerability affects Cisco devices if they are running a vulnerable
release of Cisco IOS XE Software that is configured for NAT operation and
has the DNS ALG feature enabled. The DNS ALG feature is enabled as soon as
NAT is configured on the device.
Assess the NAT Configuration
1. Determine whether a device is configured to perform NAT
Administrators can determine whether NAT is active on the device
(preferred) or whether NAT commands are present in the device
To determine whether NAT is active on a device, administrators can log in
to the device and issue the show ip nat statistics command in the CLI. If
NAT is active, the Outside interfaces and Inside interfaces sections of the
command output will include at least one interface.
The following example shows the output of the show ip nat statistics
command for a device where NAT is active:
Router# show ip nat statistics
Total active translations: 1 (0 static, 1 dynamic; 0 extended)
If the output of the show ip nat statistics command does not list any
interfaces, NAT is not active on the device.
Alternatively, administrators can determine whether NAT is active on a
device by issuing the show running-config command in the CLI and assessing
whether NAT commands are present in the device configuration. If NAT is
active on the device, the output of the show running-config command will
include the ip nat inside and ip nat outside interface commands. In the
case of the NAT Virtual Interface, the ip nat enable interface command will
2. Determine whether DNS ALG is disabled in the NAT configuration
To determine whether the DNS ALG is disabled in the NAT configuration, use
the show running-config | include ip nat service dns privileged EXEC
command. The presence of both no ip nat service dns tcp and no ip nat
service dns udp in the output of show running-config | include ip nat
service dns indicates that the DNS ALG is disabled in the NAT
The following is the output of show running-config | include ip nat in
Cisco IOS XE Software that has the DNS ALG disabled in the NAT
Router#show running-config | include ip nat service
no ip nat service dns tcp
no ip nat service dns udp
If both no ip nat service dns tcp and no ip nat service dns udp do not
appear in the output of show running-config | include ip nat service dns ,
and the device runs an affected version of Cisco IOS XE Software with NAT
enabled, that configuration is vulnerable .
Products Confirmed Not Vulnerable
Only products listed in the Vulnerable Products section of this advisory
are known to be affected by this vulnerability.
Cisco has confirmed that this vulnerability does not affect the following
IOS XR Software
o There are no workarounds that address this vulnerability. However, a
mitigation is available.
Administrators may mitigate this vulnerability by disabling the NAT ALG for
DNS packets. However, this action may negatively impact normal operation of
any device that sends or receives traffic through the affected device and,
consequently, may disrupt normal network operations.
Administrators should verify that their network environment does not
require use of a NAT ALG for DNS packets before they disable this
functionality. To disable use of the NAT ALG for DNS packets,
administrators can use the no ip nat service dns tcp and no ip nat service
dns udp command in global configuration mode.
While this mitigation has been deployed and was proven successful in a test
environment, customers should determine the applicability and effectiveness
in their own environment and under their own use conditions. Customers
should be aware that any workaround or mitigation that is implemented may
negatively impact the functionality or performance of their network based
on intrinsic customer deployment scenarios and limitations. Customers
should not deploy any workarounds or mitigations before first evaluating
the applicability to their own environment and any impact to such
o Cisco has released free software updates that address the vulnerability
described in this advisory. Customers may only install and expect support
for software versions and feature sets for which they have purchased a
license. By installing, downloading, accessing, or otherwise using such
software upgrades, customers agree to follow the terms of the Cisco
Additionally, customers may only download software for which they have a
valid license, procured from Cisco directly, or through a Cisco authorized
reseller or partner. In most cases this will be a maintenance upgrade to
software that was previously purchased. Free security software updates do
not entitle customers to a new software license, additional software
feature sets, or major revision upgrades.
When considering software upgrades , customers are advised to regularly
consult the advisories for Cisco products, which are available from the
Cisco Security Advisories page , to determine exposure and a complete
In all cases, customers should ensure that the devices to be upgraded
contain sufficient memory and confirm that current hardware and software
configurations will continue to be supported properly by the new release.
If the information is not clear, customers are advised to contact the Cisco
Technical Assistance Center (TAC) or their contracted maintenance
Customers Without Service Contracts
Customers who purchase directly from Cisco but do not hold a Cisco service
contract and customers who make purchases through third-party vendors but
are unsuccessful in obtaining fixed software through their point of sale
should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c
Customers should have the product serial number available and be prepared
to provide the URL of this advisory as evidence of entitlement to a free
Cisco IOS and IOS XE Software
To help customers determine their exposure to vulnerabilities in Cisco IOS
and IOS XE Software, Cisco provides the Cisco Software Checker to identify
any Cisco Security Advisories that impact a specific software release and
the earliest release that fixes the vulnerabilities described in each
advisory ("First Fixed"). If applicable, the tool also returns the earliest
release that fixes all the vulnerabilities described in all the advisories
identified ("Combined First Fixed").
Customers can use the Cisco Software Checker to search advisories in the
Choose the software and one or more releases
Upload a .txt file that includes a list of specific releases
Enter the output of the show version command
After initiating a search, customers can customize the search to include
all Cisco Security Advisories, a specific advisory, or all advisories in
the most recent bundled publication.
Customers can also use the following form to determine whether a release is
affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE
Software release-for example, 15.1(4)M2 or 3.13.8S.
By default, the Cisco Software Checker includes results only for
vulnerabilities that have a Critical or High Security Impact Rating (SIR).
To include results for Medium SIR vulnerabilities, customers can use the
Cisco Software Checker on Cisco.com and check the Medium check box in the
drop-down list under Impact Rating when customizing a search.
For a mapping of Cisco IOS XE Software releases to Cisco IOS Software
releases, refer to the Cisco IOS XE 2 Release Notes , Cisco IOS XE 3S
Release Notes , or Cisco IOS XE 3SG Release Notes , depending on the Cisco
IOS XE Software release.
Exploitation and Public Announcements
o The Cisco Product Security Incident Response Team (PSIRT) is not aware of
any public announcements or malicious use of the vulnerability that is
described in this advisory.
o This vulnerability was found during the resolution of a Cisco TAC support
Cisco Security Vulnerability Policy
o To learn about Cisco security vulnerability disclosure policies and
publications, see the Security Vulnerability Policy . This document also
contains instructions for obtaining fixed software and receiving security
vulnerability information from Cisco.
Related to This Advisory
o Cisco Event Response: March 2021 Semiannual Cisco IOS and IOS XE Software
Security Advisory Bundled Publication
| Version | Description | Section | Status | Date |
| 1.0 | Initial public release. | - | Final | 2021-MAR-24 |
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